Apple released new Time Machine and Airport Extreme products rather stealthily last month. There was no announcement, and even the product description pages didn't even reflect the product change right away.
While the change was small, a simple change in the wifi stack to the new Broadcam BCM4331 solution, the payoff is huge in terms of range and performance according to AnandTech.
At close range the two base stations exhibit nearly identical performance. However,as this years model was able to perform nearly at full strength where last years model barely had a signal. The new Broadcam WiFi stack uses more power, but the benefits are increased performance at greater distances from the base station.
AnandTech summed up the capabilities of the new Airport Extreme and Time Capsule products nicely:
At the end of the day, the new Airport Extreme dramatically improves throughput in the best case and in a few regions where signal was previously unusable. In the worst case (location 4), performance improves from being essentially unusable to totally fine, and in the case of the 2010MBP goes from not being able to connect at all to pushing 23 Mbps.